The present invention relates to heat-resistant cast steels, more particularly, to heat-resistant cast steels that have excellent durability (e.g. high resistance to thermal fatigue and oxidation) and that can be produced at low cost because of their good castability and machinability.
Conventional heat-resistant cast iron and cast steel include those which are shown under the heading of "Comparative Samples" in Table 1 provided herein. Automotive engine exhaust parts such as exhaust manifolds, turbocharger housings, precombustion chambers for diesel engines and parts of exhaust purifying systems are normally used under hot and hostile conditions and, to meet this operational requirement, they have conventionally been made of heat-resistant cast irons such as high-Si spheroidal graphite cast iron and Ni-Resist cast iron (see Table 1) and aluminum-alloyed cast iron, and in special cases, expensive high-alloy content heat-resistant cast steels such as austenitic stainless cast steels.
Such conventional heat-resistant cast irons and cast steels, however, have had various problems. For example, high-Si spheroidal graphite cast iron, Ni-Resist cast iron and ferritic stainless cast steels such as CB-30 (designated according to the Alloy Casting Institute Standards) insure fairly high productivity because of their good castability and machinability but on the other hand, the durability such as resistance to thermal fatigue and oxidation is so poor that it is not suitable for use in parts that is to be exposed to a temperature of not less than 800.degree. C. Aluminum-alloyed cast irons and high-alloy content heat-resistant cast steels such as austenitic stainless cast steels exhibit high durability at a temperature of 800.degree. C. or more but they are so poor in castability that defects such as shrinkage cavities and misruns are highly likely to occur during casting. These casting defects combine with the poor machinability of the aluminum-alloyed cast irons and high-alloy content heat-resistant cast steels to reduce their productivity.